1. Field of Invention
This invention relates to systems and methods for detecting and correcting image quality defects, such as banding defects, in image marking devices, such as, for example, xerographic marking devices, using feedback and/or feedforward control.
2. Description of Related Art
A common image quality defect introduced by the copying or printing process is banding. Banding generally refers to periodic, linear structures on an image caused by a one-dimensional density variation in either the cross-process (fast scan) direction or process (slow scan) direction. FIG. 1 shows an image taken from an image marking device, such as, for example, a xerographic printer that illustrates an extreme case of banding due to photoreceptor and magnetic roll runout. A typical density variation of this image in the process direction is shown in FIG. 2.
Banding defects can result due to many xerographic subsystem defects such as, for example, development nip gap variation caused by developer roll runout and/or photoreceptor drum runout, coating variations on either the developer rolls or the photoreceptor, non-uniform photoreceptor wear and/or charging, and developer material variations.
One approach to mitigate banding defects is by specifying tight tolerances in subsystem design. One problem with this “passive” approach is that stringent image quality specifications increasingly lead to subsystem components with tighter and tighter tolerances, which, in turn, are more costly to manufacture. Another potential problem is scalability. That is, the subsystem design for one product in a family may not be appropriate for a different product in the same family, thus leading to costly and time consuming redesign. Furthermore, specifying tight tolerances in subsystem design has limited robustness properties. For example, using developer rolls with a tight tolerance on runout will not help with banding due to photoreceptor wear.